Position sensors are useful for many applications in which it is necessary to determine the position and orientation of an object. Hereinafter, the term ‘position sensor’ means both position and orientation sensor. The position is the location of the object in a three-dimensional coordinate frame, and the orientation of the object is its rotation relative to that coordinate frame.
In an example application, a position sensor can be attached to a moving camera that is being used for 3D scanning. The position and orientation of the camera are then known, to aid creation of a 3D reconstruction of a scene from the images acquired by the moving camera. Similarly, a position sensor can be attached to a projector to aid correct projection of an image onto a surface or an object. A position sensor can also be attached to a tool so that the tool can be properly positioned with respect to a part or a working surface. If a user is manipulating the tool, the position sensor enables haptic or other feedback, for example in tele-operation, or for computer-aided surgery. A position sensor can also be used to track the location of a moving object such as a vehicle or person.
One way that position sensors have been used in prior art applications is to place visual markers or active emitters, such as LEDs, in known positions in the environment. In those applications, the position sensor can be a camera that observes the markers or emitters in order to infer its own position and orientation. Other applications use ultrasound instead of light. Alternatively, visual markers or active emitters can be attached to the object of interest so that the object's position and orientation can be determined directly using a camera observing the object.
A system which uses LED emitters in the environment is described in “The HiBall Tracker: High-Performance Wide-Area Tracking for Virtual and Augmented Environments,” by Welch et al, Proc. of the ACM Symposium on Virtual Reality Software and Technology, 1999. A system which uses ultrasound emitters in the environment is the IS-600 motion tracker by Intersense Corporation, Burlington, Mass.
Prior art approaches have limitations. Altering the environment can be difficult when the environment is large or includes hard-to-reach places. And the instrumentation of the environment must be followed by a calibration procedure. Thus these types of systems tend to be fixed installations with a fixed workspace, and are not easily or quickly deployable in a new setting. Systems that involve attaching markers or emitters, such as LEDs, to an object usually require multiple emitters that span the surface of the object. A calibration procedure is used to determine the placement of the markers or emitters. It is problematic to attach multiple markers or emitters to a small object. It is also problematic if the object is handheld, since the user has to adopt a grip that does not occlude the markers or emitters.
These problems could be overcome with electromagnetic sensors, but such systems are considerably more complex and costly, and calibration still remains as a problem. Most of the prior art techniques require a relatively static set-up, which makes it difficult to use such systems in ad-hoc, dynamically changing environments. An example of an electromagnetic system is the Fastrak® motion tracking product by Polhemus Inc., of Colchester, Vt.
Thus, there is a need for a position and orientation sensor for an object, which works without having to modify the environment, and which does not require a complicated calibration. There is also a need for a position and orientation sensor that is compact, i.e., the sensor does not require markers or emitters that are distributed across the environment, or across the surface of an object of interest. An advantage of compactness for a position sensor attached to a hand-held object is that the user can easily adopt a grip on some part of the object without occluding the position sensor.